[GHC] #13813: DeriveFunctor spuriously rejects existential context with type synonym mentioning the last type variable

[GHC]

#13813: DeriveFunctor spuriously rejects existential context with type synonym
mentioning the last type variable
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:  (none)
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3635
       Wiki Page:                    |
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Comment (by Ryan Scott <ryan.gl.scott@…>):

 In [changeset:"85731000d8b13476ed3c5bde22af610a27fb00f8/ghc" 8573100/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="85731000d8b13476ed3c5bde22af610a27fb00f8"
 Look through type synonyms in existential contexts when deriving Functor

 Summary:
 This amounts to using `exactTyCoVarsOfType` instead of
 `tyCoVarsOfType` in the right place. I also fixed a similar issue for
 `-XDatatypeContexts` while I was in town (but couldn't be bothered to add
 a
 test for it).

 Test Plan: make test TEST=T13813

 Reviewers: austin, bgamari, simonpj

 Reviewed By: simonpj

 Subscribers: simonpj, rwbarton, thomie

 GHC Trac Issues: #13813

 Differential Revision: https://phabricator.haskell.org/D3635
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13813#comment:2>
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